When it comes to exams, the problem is often not ignorance, but simply that we forget to remember the right things at the right time.

For essay writing in particular, there's nothing like having at your disposal a bird's-eye view of the subject matter at hand. You want to be able swiftly to look over all that could be said and pick out the most relevant parts, while leaving plenty of mental space for the crafting of an ingenious argument.

Spider diagrams are an excellent tool for creating an overview, and remembering it crisply. They help distil complex topics onto a single memorable page by using a branching spatial organisation, colour and images.

There are two fundamental reasons why spider diagrams are such a powerful tool.

The first is simple: they're straight-out memorable. Colours, pictures, and simple keywords add to a spatial structure that invites easy exploration in memory. You never get confused between the top left and the bottom right of a spider diagram in the way that you can get confused between different parts of a historical epoch, for instance. The space gives us meaning.

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The second is that their creation is active, and intertwined with the development of understanding. As we force ourselves to sift, understand and summarise in order to create the diagram, we make new connections which deepen and elaborate our understanding. And this process of making is also enacting how we'll subsequently recall them.

So, how to create one? First, take a blank sheet of paper in portrait mode. Put the essence of your subject in pictorial form in the middle of the page. This is the front door to your set of memories and thoughts – make it bright and visual and distinctive. We'll take the example of the French Revolution, and make our image a guillotine.

Next, divide your overall subject into sections, and radiate a branch for each section from the centre. Interestingly, it doesn't too much matter exactly how you slice and dice the subject, only that you do so. Here we'll divide the branches into four phases of the revolution.

Then, let's zoom in to our branch entitled The Reign of Terror 1792-95. How to elaborate this?

Well, we next add 'twigs' for the key things we might need to know about that period: the Execution of Louis XVI that began it, the Jacobins' Coup led by Robespierre, the Constitution of 1793, the terror itself, and the Thermidorian reaction that followed Robespierre's death.

Borrowing from last week's blog, it's worth chucking in a pic of a thermostat on the twig of the Thermidorian revolution, to help recall that bit.

Each of these twigs will subdivide again, with the focus on each subtopic this time. So within the Jacobin coup, the key third-level themes may be Robespierre, his mates the sans-culottes and the banishing of the Girondins from the National Assembly. So we'd have a 'leaf" for the these as well. At the leaf stage, we can go further if we like – with dates, further players etc.

What results from following this process on each branch and twig is a dense but accessible overview of a complex, interconnected subject. When you're answering some question about Robespierre, the map enables you to locate him as a key -player in the The Reign of Terror and the Jacobin coup, and to see when he was executed, and for what reason.

By zooming in, you not only recall all the relevant information arrayed around the leaves, but relive the thoughts and understanding that led you to put them there in the first place.

Ed Cooke is a Grand Master of Memory, and is co-founder and CEO of Memrise. He writes a weekly blog for Telegraph Education on revision techniques: